Cathode-ray storage tube and monitor system having controlled image persistence in nonstore mode

ABSTRACT

A system is shown for monitoring a direct viewing bistable image storage cathode-ray tube with a cathode-ray monitor tube in which the monitor tube visually displays stored writing when the storage tube is operated in its storage mode and nonstored writing when the tube is operated in a nonstore mode. The nonstored writing is displayed on the screen of the monitor tube with improved intensity and in a manner which insures that the complete image visible on the phosphor of the storage tube is reproduced on the screen of the monitor tube. This is accomplished by providing a circuit which maintains both a readout raster on the storage dielectric of the storage tube target and electron flood guns in operation while at the same time maintaining a potential on the target backplate below that necessary for the storing operation but at a value which causes the nonstored writing to have a controlled persistence lasting for at least the time duration of one frame of the readout raster.

United States Patent Inventor Charles B. Gibson, Jr.

Portland, Oreg. Appl. No. 781,759 Filed Dec. 6, 1968 Patented Aug. 17, 1971 Assignee Tektronix, Inc.

Beaverton, Oreg.

CATHODE-RAY STORAGE TUBE AND MONITOR SYSTEM HAVING CONTROLLED IMAGE PERSISTENCE IN NONSTORE MODE 5 Claims, 3 Drawing Figs.

Hon

p05 CD a e I 4 3 2 x INPUT F .Ls lTCH I04 I ivaRr [R M P05 I t CD IIZ INTENSITY 3,307,178 2/1967 Steadmanetal. 3,356,878 12/1967 Yaggyetal ABSTRACT: A system is shown for monitoring a direct viewing bistable image storage cathode-ray tube with a cathoderay monitor tube in which the monitor tube visually displays stored writing when the storage tube is operated in its storage mode and nonstored writing when the tube is operated in a nonstore mode. The nonstored writing is displayed on the screen of the monitor tube with improved intensity and in a manner which insures that the complete image visible on the phosphor of the storage tube is reproduced on the screen of the monitor tube. This is accomplished by providing a circuit which maintains both a readout raster on'the storage dielectric of the storage tube target and electron flood guns in operation while at the same time maintaining a potential on the target backplate below that necessary for the storing operation but at a value which causes the nonstored writing to have a controlled persistence lasting for at least the time duration of one frame of the readout raster.

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MODULATO TV. MONITOR PATENTED AUG! 715m CHARLES B. GIBSON JR.

l/VVE/VTUR BY BUC/(HOR/V, BLORE, KLAROU/ST 8 SPAR/(MAN ATTORNEYS CATI'IODE-RAY STORAGE TUBE AND MONITOR SYSTEM HAVING CONTROLLED IMAGE PERSISTENCE' IN NONSTORE MODE BACKGROUND OF INVENTION Direct viewing bistable storage cathoderay tubes of various types are known and in general such tubes are provided with electron flood guns in addition to a writing electron gun and its associated X- and Y-deflection plates. These tubes have a dielectric storage target which may or may not be the phosphor upon which the visual image is produced and also have a collector for secondary emission electrons which, for example, may be a mesh of conducting material between the dielectric target and the electron guns.

Storage tubes, in general, have an electrically conducting member or backplate upon which the storage dielectric is positioned. Portions of the backplate may also function as the collector and the phosphor forming the fluorescent screen upon which the visual image is produced may also be the storage dielectric. Thus a particularly effective storage tube has a phosphor layer, also constituting the storage dielectric, positioned upon a layer of transparent material constituting the backplate for the dielectric in turn positioned upon the interior surface of the glass faceplate of the cathode-ray tube. The glass plate has a large number of small raised portions covered by the transparent conducting layer protruding through the dielectric layer of phosphor so that the portions of the conducting layer protruding through the phosphor also provide an efiicient electron collector of secondary emission electrons.

Systems for operating storage tubes in the storage mode, as is the case in the system disclosed herein, conventionally cause a beam of high velocity electrons from a writing electron gun to strike the storage dielectric while the flood guns are in operation to flood the surface of the storage dielectric with lower velocity electrons. The voltage between the backplate and the cathodes of the flood guns is such that the ratio of the number of secondary emission electrons from nonwritten areas of the storage dielectric to the number of primary electrons from the flood guns striking these areas is less than unity. Under these conditions the surface of the unwritten areas of the phosphor approaches the potential of the flood gun cathodes and the velocity with which the electrons from the flood guns strike the phosphor in these areas is very low so that little fluorescence, is produced.

The voltage between the backplate and the cathode of the writing gun is, however, such that the ratio of the number of secondary emission electrons from the elemental areas of the storage dielectric struck by the writing beam to the number of primary electrons striking such areas is greater than unity so that the written areas assume a positive potential with respect to the nonwritten areas. The voltage referred to above between the backplate and the cathodes of the'flood guns is, however, well within the bistable storage range in which the electrons from the flood guns strike the more positive written areas with sufficient velocity that'the ratio of secondary emission electrons to primary electrons remains greater than unity. This causes the more positive elemental areas originally struck by the electrons of the writing beam to tend'to assume a maximum positive voltage relative to the cathodes of the flood guns determined by the voltage between the'backplate and the cathodes of the flood guns. Thus thevelocity with which the flood gun electrons strikethe phosphor is sufficient to continue and in most cases increase the original brightness of the fluorescence produced by the electrons of the writing gun' striking the phosphor.

The stored image on the storage dielectric of the target may be reproduced upon the screen of a monitor cathode-ray tube 7 raster in the monitor tube by a writing gun and deflection system-The intensity of the" readout electron beam in the storage-tube is maintained sufficiently low so that storage of theraster does not occur and so that'the background illumination of the target of the storage tube also remains low.

An electron curre'ntha'ving one value when the electron beam of the readout electron gun strikes an unwritten po'rtion of the storage target'dielectric and another value when this beam strikes a more positive written portion can be'obtained from any one of several electrodes in the storagetube.- It is convenient to derive a signal from the electron current in the external connection to a collector of secondary emission'electrons closely adjacent the storage dielectric. In the tube specifically described above, this signal can be derived from current flowing from the smallcollector elements protruding through the storage dielectric, through the backplate and the external connection to the backplate. Variations in this current can be employed to supply Z-axis modulation to the monitor tube to reproduce on the screen of the monitor tube an image corresponding to the stored image of the storage tube.

Operation of the above system in the bistable storage mode cannot, however be employed to follow a change in'the image being displayed on either of the two tubes unless frequent erasing operations are also carried out. Such erasing operations cause unpleasant blinking of both the storage tube and the monitor tube. The erasing operations are required since any change in the image being written by the writing beam will leave the originally stored image as a visual as well as an elec-,

trical image in its original position and any other portions of the target struck by the writing beam from the writing gun during the change will be stored and appear as part of the visual image on both tubes. Changes in form or size of the image being displayed either as a result of a change in the signal supplied to the storage oscilloscope or as a result of manual adjustment of the signal attenuating or position or sweep speed controls of the oscilloscope, will result in the fluorescent screen becoming covered with broad bands of illumination instead of a display of the desired signal.

The storage tubes above discussed have also been conventionally operatedin a nonstore mode by rendering the flood guns inoperative, for example, by providing a switch for disconnecting the cathodes of the flood guns from the negative terminal of the voltage supply circuit connected between these cathodes and the collector for the storage target. The storage tube then'becomes'the equivalent of a conventional oscilloscope tube and for all but the very lowest writing speeds thepersistance of the phosphor of the storage tube is sufficient to provide a complete visual image of any signal being written.

Employingthe monitoring system above discussed with the storage tube in the no'nstorage mode obtained by disconnectingthe flood gun-cathodes does not in many cases result in a 7 complete reproduction on the screen of the monitor tube of the image-being written on the target of the storage tube. Thus the'persista'nce of the image on the phosphor or other storage dielectric is not sufficient'to insure that the electron beam of the'readout gun in following the readout raster will always strike'an area written by the writing gun before this written area fades to a potential-which will not provide a change in collector current sufficient to cause writing of the corresponding area on the screen of the monitor tube. The result is that the image on the screen' of the monitor tube will, in general, be fragmentary. This means that 'theconventional nonstore mode can also not be satisfactorily employed to follow on the monitor tube a change in the visual display on the target of the storage tube.

SUMMARY OF INVENTION In accordance with the present invention it has been found that an improved 'nonstorage mode operation of storage tube may" be obtained which enables following on the screen of a monitor .tube a changing visualimage on the target of the storage tube.

Instead of rendering the flood guns of the storage tube inoperative to provide a nonstore mode, it has been found that the voltage between the backplate of the storage target and the flood gun cathodes can be reduced to a value which will not cause bistable storage, but which will produce a controlled persistance of the storage dielectric of the target for an interval which is at least as great as the time duration of one frame of the readout raster. This makes it possible for the monitor tube to always show the complete visual image appearing on the storage tube phosphor at any time and to follow any changes in such image.

Any image stored on the phosphor of the storage tube, when the operating mode is changed from store to nonstore, gradually fades as is also the case of any image written during the nonstore mode, if the writing of such image is discontinued or moved to some other portion of the display on the storage tube. In the latter event the effect on the visual display of the monitor tube as well as that of the storage tube is that the complete image referred to above moves from one position to another. No erasing operations are required.

BRIEF DESCRIPTION OF THE DRAWINGS FIG. 1 is a block diagram, which is partly schematic and partly diagrammatic, of a system in accordance with the present invention including a direct viewing storage cathode ray tube employing the same electron gun for both storage and readout;

FIG. 2 is a fragmentary cross-sectional view on a very much enlarged scale of the storage target structures of the storage cathode-ray tube of FIG. 1; and

FIG. 3 is a fragmentary elevational view of the target shown in FIG. 2.

DESCRIPTION OF THE PREFERRED EMBODIMENTS A direct viewing storage cathode-ray tube having a single electron beam gun 12 for both writing on a storage dielectric of a direct viewing storage target 14 and reading out information being displayed on target 14 is shown in FIG. 1. The electron gun 12 may be of any conventional type having a cathode I6, intensity control grid 18, accelerating anode 20, and horizontal and vertical deflection plates 22 and 2 4, respectively. Such a storage tube will also usually have a plurality of electron flood guns connected in parallel, one of which is shown at 26 and a second acceleration anode 28.

A particularly effective form of storage target 14 is shown in FIGS. 2 and 3 and includes a transparent glass plate 30 forming the faceplate of the tube 10. The interior surface of the plate 30 is etched or otherwise formed to provide a plurality of spaced protrusions 32, and this surface including the surfaces of the protrusions is covered by a thin coating of a transparent electrically conducting material 34 such an an evaporatively deposited coating of tin oxide. A thin layer of phosphor 35 is deposited on the surface of the glass plate around the coated protrusions 32 to provide both the storage dielectric of the target and a fluorescent screen visible through the coating 34 and the glass plate 30. The portions of the coating extending above the phosphor on the protrusions 32 function as the major collectors of secondary emission electrons from the phosphor as the result of high velocity electrons striking the phosphor. The resulting electron current flows through the remainder of the conductive coating and an external connection 37 to such coating, this remaining portion also being the conductive backplate for the phosphor.

The target 14 just described is of known construction except that care must be taken to provide a phosphor layer having electrical and physical characteristics including thickness, particle size, dielectric and fluorescent properties which are extremely uniform throughout the storage and viewing area. Otherwise the persistence of the electrical and visual image on the phosphor storage dielectric in the nonstore mode discussed above will not be sensibly uniform over the area of the dielectric and the image will fade at different rates on different portions of such area.

In general, the cathode 36 of the flood gun 26 will be maintained at or near the chassis ground potential, the backplate 34 of the target 14 will be maintained approximately 150 volts positive while the tube is operating in the storage mode and the cathode 16 of the electron gun 12 will be maintained at a negative potential of, for example, about 3,000 volts. The control grid 18 is maintained at a proper average negative bias with respect to the cathode 16 by a high voltage negative bias supply 38 interposed between the grid 18 and an output Z-axis amplifier 40. The horizontal and vertical deflection plates 20 and 22 usually have an average or zero deflection potential approaching chassis ground potential and are supplied with deflection voltages by x and y axes output amplifiers 42 and 44 respectively.

The average voltage on the backplate 34 of the target 14 is shown as being applied through an amplifier 46 supplied with both an input from a store and nonstore control system 47 indicated as including a pair of screwdriver adjustable potentiometers 48 and 50 having their resistance units connected between chassis ground and a positive potential. The adjustable contacts of the potentiometers are connected to two terminals of a switch 52 so that the adjustable contacts can be alternatively connected to an input of the amplifier 46.

The potentiometer 48 is adjustable to set the average potential of the backplate 34 of the target 14 at the storage potential, which will usually be approximately 150 volts positive, when the switch 52 is in the store position and this potential will be at a lower positive value, for example approximately volts, when the switch is in the nonstore position. The movable contact of the potentiometer 48 is adjusted to provide optimum storage characteristics and the movable contact of the potentiometer 50 is adjusted to provide a positive potential on the backplate 34 of the target 14 which is just below that at which bistable storage occurs, but which is suffcient to produce a controlled persistence of the image upon the storage dielectric or phosphor.

The output impedance of the amplifier 46 provides a load for electron flow caused by the collection of secondary emission electrons by the backplate 34 and variations in voltage due to variations in the amount of secondary emission of electrons from the target dielectric will be applied to the input of a readout amplifier 54 through a capacitor 56.

A standard type of erase pulse may also be applied to the backplate 34 of the target 14 through the amplifier 46 from an erase pulse forming circuit 58 under control of an initiating pushbutton switch 60, shown adjacent the erase circuit block. This may be a positive pulse to cause a fade positive, i.e. cause the flood guns to furnish electrons of sufficient velocity to write the entire surface of the phosphor to a stored condition, followed by a negative pulse which is allowed to gradually decay to the store potential set by the potentiometers 48 of the store or nonstore circuit 47. This produces a fade negative" of the surface of the dielectric phosphor of the target exposed to the flood gun electrons to restore this entire surface to the nonwritten condition.

During the entire erase procedure just described, a gating pulse of the right polarity to disable the amplifier 54 may be applied to this amplifier from the erase circuit 48 to prevent rapid changes of illumination of the entire screen of the monitor tube. Nevertheless, this erase operation causes undesired blinking of the visual target of both the storage tube 10 and any monitor tube supplied with a Z-axis signal from the target 14 of the tube 10 through the readout amplifier 54. It will be understood that the erase operation will ordinarily be employed only when the storage tube 10 is being operated in the storage mode and it is desired to quickly remove a stored image from the target 14 of such tube.

In order to employ the same electron gun'- 12 of the storage tube 10 for both writing and readout, the system of FIG. I shows a time sharing switching system which includes electronic switches 62, 64 and 66 for the x, y and z axis inputs to the amplifiers 42, 44 and 40 respectively, for enabling the gun 12 to write;on the target only during the horizontal flyback or return trace time of the readout raster discussed above. For example, a conventional television raster may be employed for the readout. For rapid writing speeds, one or more traverses by the writing beam across the entire target may occur during this flyback time and for slower writing speeds the writing is in the form of a series of dashes or dots which, however, are produced at a sufficiently rapid rate to make the writing appear continuous on the storage tube display.

A raster may be produced by a system including a synchronizing oscillator 68 supplying pulses to a pulse counter 70 which delivers ramp initiating pulses in proper times relation to a switch driver 72 which in turn delivers ramp initiating voltages to an X-ramp or horizontal deflection voltage generator 74, supplying an X-ramp or horizontal saw tooth sweep voltage to an input of the x ramp switch 62. The counter 70 also supplies properly timed ramp initiating voltages to a Y- ramp or vertical deflection voltage generator 76 which in turn supplies a Y-ramp or vertical saw tooth deflection voltage to an input to the y switch 64. In the read position of the electronic switches 62 and 64, horizontal and vertical deflection voltages from the ramp generators 62 and 64 are supplied through the amplifiers 42 and 44, respectively, to the deflection plates 22 and 24, respectively, of the tube 10.

The Z-axis electronic switch, when in the read position, supplies a read intensity voltage from a read intensity control circuit 78, indicated as having a screwdriver control 79 for setting the value of the intensity control voltage, to the control grid 18 of the electron gun 12 through the amplifier 40 and the negative bias circuit 38, the bias circuit 38 adding a constant negative voltage of the order of 3,000 volts, for example, to any voltage received from the amplifier 40.

The switches 62, 64 and 66 also have inputs from x, y and z preamplifiers 80, 82 and 84, respectively. These amplifiers receive x, y and 2 inputs 86, 88 and 100, respectively. The x input 86 may, for example, be from the sweep circuit of an oscilloscope containing the storage tube or may be from an external source. Similarly, the y input 88 may be from the vertical signal attenuator circuits of the oscilloscope or from an external source, and the 2 input may be from the unblanking circuit of the oscilloscope or from an external source of modulating or blanking voltage. This, for example, all three inputs 86, 88 or 100 may be from a character generating source or from a source supplying electrical information for drawing a graph or sketch, etc.

The amplifiers 80, 82 and 84 may be cathode coupled amplifiers having two separate inputs. These amplifiers also accept horizontal position, vertical position, and intensity control voltages, respectively, from a vertical position circuit 102, a horizontal position control circuit 104 and a write intensity control circuit 106, each voltage being manually adjustable by an associated control knob 108, 1 10 or 112, respectively.

The voltages from the switch driver 72 are also supplied as operating voltages to the switches 62, 64 and 66 to cause these switches to connect the inputs from the X-ramp generator 74, Y-ramp generator 76 and read intensity circuit 78 to the amplifiers 42, 44 and 40, respectively, during the generation of each X-ramp or saw tooth horizontal deflection voltage by the X-ramp generator and to connect the inputs of the amplifiers 80, 82 and 84 to the amplifiers 42, 44 and 40 during the fly back or horizontal trace return. This means that the readout raster producing circuits including the oscillator 68, counter 70, switch driver 72 X-ramp generator 74, Y-ramp generator 76 and readout intensity control circuit 78 are in control of the electron gun l2 and deflection plates 22 and 24 of the tube 10 whenever a ramp portion of a horizontal deflection voltage of the readout raster is being produced. It also means that the inputs to the preamplifiers 80, 82 and 84 are in control of the electron gun l2 and the deflection plates 22 and 24 during the retrace times of the horizontal deflection of the readout raster. This effectively blanks any writing of the retrace on the target 14 of the tube 10, but preferably short blanking pulses are sent to the amplifier 40 from a blanking circuit 114 under control of voltages supplied from the counter 70 to cause blanking of 6 the electron beam of the electron gun 12 during the switching operations of the switches 62, 64 and 66. I v

A sync pulse generator. for supplying standard television horizontal and verticalsynchronizing pulses to the'read amplifier 54 may also becontrolled by voltages from the counter 70 and switch driver 72. These pulses are addedto the Z-axis signal from the backplate 34 to produce a" standard black and white video signal which may be employed to modulate a radio frequency carrier by the RF generator andtmodulator 116 to produce a standard television signal which is delivered to monitor 118, which may be a standard television set.

It will be apparent that the video signal from the read amplifier 54 can be delivered directly to the video circuits of a monitor which has no radio frequency circuits or in a manner bypassing such circuits, and that it is also possible to employ a monitor which itself has no circuits for producing a raster by delivering the outputs from the X- and Y-ramp generators 74 and 76, respectively, directly to the monitor, in which case the sync pulse generator 115 would not be employed. In any of these variations the display on the monitor screen will be the same, and as stated above the controlled persistence of the storage dielectric or phosphor 35 causes all portions of the image written by the beam from the electron gun 12 during the retrace intervals of the readout raster to persist for at least the time duration of one raster. This enables the display on the fluorescent screen of the monitor 118 to accurately follow changes in the image displayed on the fluorescent screen of the storage tube provided by the phosphor 35. Although a reduction of the positive potential of the backplate from that causing storage of the image is the disclosed operation for securing the controlled persistence of the image written upon the storage dielectric, it is the potential difference between the cathodes of the flood guns and the backplate which is the controlling factor, and it is therefore apparent that the controlled persistence can also be produced by suitably changing the potential applied to these cathodes.

While a direct viewing storage cathode-ray tube 10 having a single electron gun 12 for both writing on the storage dielectric or phosphor. 35 and reading out the image displayed thereon, it is apparent that a similar tube having separate writing and readout electron guns can be employed, in which case the switches 62, 64 and 66 would be eliminated and a gating or unblanking voltage supplied to the control grid of the writing gun so that writing by this gun would be restricted to the retrace intervals between the horizontal lines of the readout raster. That is to say, the outputs of the preamplifiers 80, 82 and 84'would be supplied directly to the writing gun through appropriate output amplifiers and the outputs of the ramp generators 74 and 76 and the read intensity circuit 78 similarly supplied to the readout gun.

I claim:

1. An image storage cathode ray tube and'visual monitor tube system, comprising:

a cathode-ray tube having electron beam and electron flood guns and a target provided with a storage dielectric supported upon a backplate of electrically conducting material and positioned to receive electrons from said guns;

means including a deflection circuit for causing a readout electron beam from one of said guns to produce successive readout patterns on said dielectric;

means for causing a writing electron beamfrom one of said guns to write an electric image on said dielectric;

said means for producing a readout pattern including horizontal and vertical deflection means for producing a readout pattern in the from of a raster and said system having means for restricting the writing by said writing beam to the horizontal retrace intervals of the horizontal deflection means;

means for providing a preset store mode positive potential of said backplate relative to the cathodes of said guns which causes storage of said electrical image on said dielectric;

means for also providing a preset nonstore mode lower positive potential of said backplate relative to flood gun cathode potential which will not cause said storage but which is sufficient to produce a controlled persistence of said image for a time interval at least as great as the duration of one of said readout patterns;

switching means for switching between said store mode positive potential and said nonstore mode lower positive potential to alternatively select said store mode and said nonstore mode providing said controlled persistence;

and a visual display cathode-ray monitor tube and means responsive to variations in emission of secondary electrons from said dielectric and to signals from said deflection circuit to cause said monitor tube to visually reproduce said image.

2. The system of claim 1 in which the means for alternatively providing a nonstore mode includes means for reducing the potential of the backplate with respect to the cathodes of said guns.

3. The system of claim I in which the backplate is a transparent layer on a transparent faceplate of said tube, the storage dielectric is a phosphor which also provides a visible fluorescent image of the electrical image written thereon and the means for providing a nonstore mode applies a backplate to flood gun cathode potential during said nonstore mode which produces a controlled persistence of said electrical image causing the visual image on said monitor tube to accurately follow the visible image on said phosphor.

4. The system of claim 3 in which the electrical characteristics and thickness of said phosphor is sufficiently uniform throughout the area of said phosphor on said backplate that the controlled persistence of said electrical and visual image is substantially the same for all portions of said phosphor.

5. The system of claim 1 in which the same electron beam gun is employed to provide both said writing electron beam and said readout electron beam and said system includes electronic switching means responsive to said means for producing said readout pattern for switching the control of said same electron beam tube to said means for producing said readout raster from said writing beam control means at the start of each horizontal deflection of said readout beam during said raster and back to said writing beam control means from said means for producing said readout raster at the end of each said horizontal deflection.

UNITED STATES PATENT OFFICE CERTIFICATE OF CORRECTION Patent No. 50 D t August 17 1971 Inventor(s) Charles B. Gibson, Jr.

It is certified that error appears in the above-identified patent and that said Letters Patent are hereby corrected as shown below:

I Column 4, lines 10 and 11, "20 and 22" should read --22 and 1 24--. Column 5, line 21, "62 and 64" should read --74 and 76--. Column 6,- line 42, after "thereon" insert --has been described--. Column 8, line 16, "tube" should read --gun--. Column 8, line 17, "raster" should read --pattern--.

Column 8, line 19, "raster" should read -'-pattern--. Column 8, line 20, "raster" should read --pattern--.

Signed and sealed this 29th day of February 1972.

(SEAL) Attest:

EDLJARD FLFLETC HER, JR R0 BERT TT CIL L Ii Atbesting Officer Commissioner of Patents 

1. An image storage cathode ray tube and visual monitor tube system, comprising: a cathode-ray tube having electron beam and electron flood guns and a target provided with a storage dielectric supported upon a backplate of electrically conducting material and positioned to receive electrons from said guns; means including a deflection circuit for causing a readout electron beam from one of said guns to produce successive readout patterns on said dielectric; means for causing a writing electron beam from one of said guns to write an electric image on said dielectric; said means for producing a readout pattern including horizontal and vertical deflection means for producing a readout pattern in the from of a raster and said system having means for restricting the writing by said writing beam to the horizontal retrace intervals of the horizontal deflection means; means for providing a preset store mode positive potential of said backplate relative to the cathodes of said guns which causes storage of said electrical image on said dielectric; means for also providing a preset nonstore mode lower positive potential of said backplate relative to flood gun cathode potential which will not cause said storage but which is sufficient to produce a controlled persistence of said image for a time interval at least as great as the duration of one of said readout patterns; switching means for switching between said store mode positive potential and said nonstore mode lower positive potential to alternatively select said store mode and said nonstore mode providing said controlled persistence; and a visual display cathode-ray monitor tube and means responsive to variations in emission of secondary electrons from said dielectric and to signals from said deflection circuit to cause said monitor tube to visually reproduce said image.
 2. The system of claim 1 in which the means for alternatively providing a nonstore mode includes means for reducing the potential of the backplate with respect to the cathodes of said guns.
 3. The system of claim 1 in which the backplate is a transparent layer on a transparent faceplate of said tube, the storage dielectric is a phosphor which also provides a visible fluorescent image of the electrical image written thereon and the means for providing a nonstore mode applies a backplate to flood gun cathode potential during said nonstore mode which produces a controlled persistence of said electrical image causing the visual image on said monitor tube to accurately follow the visible image on said phosphor.
 4. The system of claim 3 in which the electrical characteristics and thickness of said phosphor is sufficiently uniform throughout the area of said phosphor on said backplate that the controlled persistence of said electrical and visual image is substantially the same for all portions of said phosphor.
 5. The system of claim 1 in which the same electron beam gun is employed to provide both said writing electron beam and said readout electron beam and said system includes electronic switching means responsive to said means for producing said readout pattern for switching the control of said same electron beam tube to said means for producing said readout raster from said writing beam control means at the start of each horizontal deflection of said readout beam during said raster and back to said writing beam control means from said means for producing said readout raster at the end of each said horizontal deflection. 